It is commonly stated in the literature on laminar flow control (see, for example, ref. 1) that the important parameter controlling the height of tolerable roughness is the Reynolds number per foot, or unit Reynolds number, Vh; provided this parameter is the same in any two cases (which are closely geometrically similar) then it is assumed that the tolerable roughness height will also be similar. It is the object of the present note to point out that this is true only in very restricted circumstances and that in general the actual physical scale is also of considerable importance, as the following shows.